Coal-based integrated gasification combined cycle plant (IGCC) technology enables production of electricity with a gas turbine utilizing a fuel that is rich in hydrogen and has a very limited amount of CO2. Combustion of the hydrogen-containing fuel requires an oxidizing source such as air, which contains nitrogen (N2). As a result, a by-product in exhaust gas stemming from hydrogen-containing fuel combustion is a significant amount of NOx. The exhaust gas further comprises significant amounts of H2O, O2, and SO2. The NOx in the exhaust gas may be reduced by using selective catalytic reduction (SCR) systems along with low NOx combustors in the gas turbine. Since fuel produced and used at an IGCC plant contains hydrogen (H2), the fuel may also provide hydrogen as a reducing agent in the SCR process by introducing a small amount of H2 from the fuel supply into the SCR system. The use of hydrogen as a NOx reducing agent enables the elimination of typical reducing agents such as, for example, ammonia (NH3) and urea (N2H6CO) in the SCR system, and thus prevents discharge of ammonia slip into the ambient air, which is an inherent problem with current ammonia-based SCR technology.
Reduction of NOx using H2 has the potential to generate reaction products that include both N2 and N2O. Catalysts that display high selectivity towards the formation of N2 are preferred. Conversely, it has been found that the selectivity of Pt-based H2—SCR catalysts toward N2 formation is relatively low, and undesirable by-products such as N2O and NH3 are produced. Recently, an attempt to improve H2—SCR efficiency with respect to NOx removal and N2 selectivity under oxidizing conditions was made (U.S. Pat. No. 7,105,137). The developed Pt-based catalyst described in U.S. Pat. No. 7,105,137 remained durable for only 24 hours when operating in a reaction mixture that contained 5 vol. % O2, 5 vol. % H2O, and up to 25 ppmv of SO2. In addition, M. Machida et al. (Applied Catalysis B: Environmental 35 (2001) 107) demonstrated that a Pt-based H2—SCR may have high selectivity to N2 under oxidizing conditions (10 vol. % O2) in the absence of H2O and SO2 in the process stream. However, demonstrations of the H2—SCR's ability to efficiently reduce NOx emissions were performed using mixtures of gases that have relatively low concentrations of O2, H2O, and SO2 or high concentrations of only one of these constituents (O2, H2O, or SO2). The exhaust gas composition of Machida et al. is contrasted to a gas turbine exhaust mixture from combustion of H2-containing fuels at IGCC plants, which contain up to about 10% O2, up to about 20% H2O and up to about 25 ppmv SO2. Importantly, catalyst systems are known by one skilled in the art to be particularly prone to deactivation and degradation over time due to exposure to sulfur compounds and/or high concentrations of water vapor.
It is known by those skilled in the art that H2—SCR is an efficient technology in O2-lean conditions, especially when amounts of water and sulfur compounds are limited to less than about 5 vol. % and to less than about 5 ppmv, respectively.
Commercial processes, such as Selexol™, may remove greater than 97% of the sulfur from syngas. Still, the concentration of sulfur compounds in syngas can be up to about 25 ppmv even after treatment. Taking into consideration dilution of syngas with nitrogen, the concentration of SO2 in IGCC gas turbine exhaust can be at the level of up to about 10 ppmv. After CO2 sequestration and burning of H2-fuel, concentrations of H2O in the exhaust can be as high as 20-25% by volume, and oxygen content can be as high as about 10-12 vol. %. Under these conditions, developing a process to reduce NOx emissions in the exhaust of IGCC gas turbines by using H2—SCR is challenging. Thus, despite the above-described enhancements, there is a need to develop processes to reduce NOx emissions in gas turbine exhaust utilizing an H2—SCR that provides high NOx reduction efficiency at the level of 90+% with high (greater than about 80%) selectivity to N2. Additional processes are needed to substantially extend durability and stability of catalyst systems in the presence of 10-25 vol. % of water, 5-10 vol. % of O2, and 5-25 ppm of SO2, for example.